Regular leisure-time exercise is a key contributor to health whereas a sedentary lifestyle is cited as one of the main causes of the observed rise in BMI and its related disorders. Despite its well-known benefits, regular leisure-time exercise behavior drops from childhood to adolescence and reaches unacceptable low proportions in adulthood, with the majority of people not engaging in regular exercise at the recommended level. Current intervention programs still largely adopt a 'one-size fits all' strategy that assumes that the determinants of leisure-time exercise behavior are the same across all adolescents, and that they are mostly of social and environmental origin. This ignores the overwhelming recent evidence that genetic factors also play an important role in voluntary exercise behavior. We hypothesize that genetic effects on exercise ability and the acute mood responses to exercise are key determinants of adolescent exercise behavior. To test this hypothesis, we propose a combined survey, laboratory, and intervention study using the unique data collection in the Netherlands Twin Register (NTR). The NTR is the largest genetically informative database in the world with longitudinal assessment of exercise behavior and many of its potential (environmental) moderators at ages 7, 9, 12, 14, 16, and 18. We aim to quantify and identify the causes of individual differences in voluntary exercise behavior from childhood to young adulthood. To this end we propose to use a longitudinal twin-family study to determine the relative contribution of genetic and environmental factors to the changes in exercise behavior over the critical period from age 7 to age 18. We furthermore set up a laboratory protocol in order to test the hypothesis that individual differences in exercise ability and the psychological response to exercise are the major factors underlying heritability of adolescent exercise behavior. The core idea behind the grant is that an increased understanding of the genetic and environmental factors underlying the voluntary choice to exercise will yield better personalized intervention strategies. To provide proof of principle, we will use the information obtained in the work described above to develop a novel family-based E- intervention program to change exercise behavior in young adolescents. Feasibility and short and long-term effects of this program will be tested in a randomized controlled trial.